Balb/c H-2db spleen cells generated a strong primary in vitro CML response against Balb/c (H-2d) with type spleen cells. The effector cells generated were specific for L antigens. In an allogeneic CML response between two non-mutant strains directed against H-2D region antigens, two clones of cytotoxic effector cells could be identified, one with specificity for D antigens, and the second with specificity directed against L antigens. When syngeneic cytotoxic effector cells were generated against TNP modified H-2D antigens, no clones with specificity for L-TNP modified antigens could be detected. Therefore, although L alloantigens are capable of inducing allogeneic cytotoxic effector cells, these antigens may not be capable of elliciting syngeneic effector cells specific for TNP modified antigens.